Superplastic forming processes are known in the art to be a viable commercial method of forming metals beyond the limitations of conventional sheet metal forming processes. Superplastic sheets of metals are generally deformed by a single sided gas pressure applied against the sheet of metal positioned above a die cavity. A pure inert gas is used for pressurization and used to prevent oxidation or impurity contamination of the sheet metal during the pressurized forming process. Superplastic sheet metal, at an elevated temperature, is disposed above the die cavity with a gas pressure directed against the sheet metal towards the die cavity so to deform the sheet metal into a part defined by the die cavity topography.
Two phase materials with a stable fine grain size and with a grain growth impedance component, such as Ti-6AL-4V at their superplastic temperature, exhibit superplastic forming characteristics. These sheet metal materials typically have low flow stresses at high temperatures suitable for superplastic deformation. The superplastic material is elongated at relatively low strain rate preventing excessive and variable thinning or premature rupturing of the material during the formation of complicated parts. However, a low strain rate decreases the speed at which the superplastic material is deformed during the forming process.
The material defines the superplastic temperature at which the sheet metal is deformed. A gas pressure versus time profile applied against the sheet metal is critical to the economic success of the forming process given a particular die cavity topography. As the time of the forming process is reduced, the total cost per part is reduced. Increased pressure upon a deforming superplastic material increases the deformation rate. Thus, a lower and longer pressure versus time profile of a superplastic deformation process increases the cost of each formed part.
In determining the pressure versus time profile for a given die cavity topography, two dimensional models were developed. The two dimensional models are used to approximate the form of the material during the forming process. However, the die cavity is a three dimensional form. As such, the two dimensional models and corresponding equations are only a gross approximation of the actual form of the material during the forming process. Consequently, the pressure versus time profile generated by equations derived from the two dimensional model are grossly inaccurate and conservative resulting in increased processing time for a particular formed part.
The two dimensional equations also did not take in to account other real phenomena which occur during the forming process. The two dimensional models and equations used to develop the pressure versus time profile did not include the die cavity surface friction. Die cavity surface friction relates to the material moving tangentially against the surface of the die cavity during the forming process.
Variable flow stress relates to the flow of metal as the material expands and elongates into the die cavity during the forming process. The two dimensional models and the corresponding equations were developed to generate the pressure versus time profile which equations did not include the effects of the variable flow stress of the superplastic material during deformation, but rather assumed for computational purposes that the flow stress was a constant. An equation using a constant flow stress is not as accurate as one using a variable flow stress.
The exclusion of the die friction effects and the exclusion of variable flow stress effects further reduces the accuracy of the two dimensional models and the derived equations in terms of providing an accelerated pressure versus time profile and further reduces the ability to predict the thickness at any point on the material and at any time during the forming process.
To avoid rupturing and excessive thinning, the two dimensional models and corresponding equations were generally conservative. The two dimensional model and the corresponding equations used a relatively low strain rate. Consequently, the time to form a particular part was relatively long. Also, the equations could not predict with much accuracy the thickness of the part at any point during the forming process. These and other disadvantages are reduced using an improved superplastic forming method which includes the die friction effects, the variable stress effects and based upon three dimensional models.